linux-kernel - Re: [PATCH 1/3] mm/slub: directly load freelist from cpu partial slab in the likely case

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Message-ID: <36964450-f45a-4f35-a187-dc493246ef59@bytedance.com>
Date: Tue, 23 Jan 2024 10:51:17 +0800
From: Chengming Zhou <zhouchengming@...edance.com>
To: Vlastimil Babka <vbabka@...e.cz>,
 "Christoph Lameter (Ampere)" <cl@...ux.com>
Cc: Hyeonggon Yoo <42.hyeyoo@...il.com>, Joonsoo Kim
 <iamjoonsoo.kim@....com>, Pekka Enberg <penberg@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] mm/slub: directly load freelist from cpu partial slab
 in the likely case

On 2024/1/23 01:13, Vlastimil Babka wrote:
> On 1/19/24 04:53, Chengming Zhou wrote:
>> On 2024/1/19 06:14, Christoph Lameter (Ampere) wrote:
>>> On Thu, 18 Jan 2024, Chengming Zhou wrote:
>>>
>>>> So get_freelist() has two cases to handle: cpu slab and cpu partial list slab.
>>>> The latter is NOT frozen, so need to remove "VM_BUG_ON(!new.frozen)" from it.
>>>
>>> Right so keep the check if it is the former?
>>>
>>
>> Ok, I get it. Maybe like this:
> 
> I think that's just too ugly for a VM_BUG_ON(). I'd just remove the check
> and be done with that.

Ok with me.

> 
> I have a somewhat different point. You reused get_freelist() but in fact
> it's more like freeze_slab(), but that one uses slab_update_freelist() and
> we are under the local_lock so we want the cheaper __slab_update_freelist(),
> which get_freelist() has and I guess that's why you reused that one.

Right, we already have the lock_lock, so reuse get_freelist().

> 
> However get_freelist() also assumes it can return NULL if the freelist is
> empty. If that's possible to happen on the percpu partial list, we should
> not "goto load_freelist;" but rather create a new label above that, above
> the "if (!freelist) {" block that handles the case.
> 
> If that's not possible to happen (needs careful audit) and we have guarantee

Yes, it's not possible for now.

> that slabs on percpu partial list must have non-empty freelist, then we
> probably instead want a new __freeze_slab() variant that is like
> freeze_slab(), but uses __slab_update_freelist() and probably also has
> VM_BUG_ON(!freelist) before returning it?
> 

Instead of introducing another new function, how about still reusing get_freelist()
and VM_BUG_ON(!freelist) after calling it? I feel this is simpler.

Thanks!